Aim: To determine the peculiarities of changes in the levels of B vitamins and their dependence on body mass index (BMI) in patients with GERD and metabolic dysfunction-associated fatty liver disease (MDAFLD). Materials and Methods: 112 patients with GERD were examined. Patients were divided into two groups. Group 1 included 50 patients with GERD, and group 2 consisted of 62 patients with GERD in combination with MDAFLD. The level of homocysteine and B vitamins in the blood serum was determined. Results: At patients group 1 were diagnosed the decreased levels of B6 and B12 in blood serum against a background of a slight increase in homocysteine levels in blood serum (up to 17.9±0.7 umol/l – p<0.05). In patients in group 2, a significant decrease in all B vitamins examined in the blood serum was found, accompanied by a significant increase in homocysteine levels (2.4 times – p<0.01). Conclusions: In the vast majority of patients with GERD and MDAFLD, an increase in BMI was found, namely, overweight in 25.8% (p < 0.01) of patients and grade I obesity (in 37.1% of patients – p < 0.001). In patients with GERD and MDAFLD, a significant decrease in serum levels of B vitamins (B1, B3, B6, B9, B12) was found, which correlates negatively with overweight and obesity in these patients.